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Re: Dissimilar Metals

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If available "electrolyte" cannot get through, around, or connect, the gasket/pad, I wouldn't expect there to be any significant "arc-ing", or galvanic activity.  If the connection is expose to weather etc, possible electrolytic paths would be a concern.  Otherwise, any required distance will probably be a function of the resistivity of the gasket/pad.
 
I don't know the chemistry of "separated" metals where there is no path.  Potentials may well be set up that affect the interaction of those very metals to other adjacent objects.
 
You might want to ensure there is sufficient "edge" distance as well. 

Thor A Tandy  P.Eng, MIPENZ
Victoria BC
Canada
e-mail: vicpeng(--nospam--at)telus.net
 
----- Original Message -----
Sent: Thursday, August 08, 2002 4:14 PM
Subject: Dissimilar Metals

To All -
  I have a situation where, via a bolted connection, an aluminum vertical
plate is attached to a steel shear tab located along the web of of steel
channel. I specified a 1/2" thick closed cell neoprene pad (plus S.S. bolts)
between the aluminum plate and the shear tab. Is there a rule of thumb or
guidance regarding the minimum separation distance between dissimilar
materials? I know in similar cases, the aluminum plate could be coated with
an asphaltic paint where it abuts the shear tab so perhaps no minimum
separation is required. I'm concerned that should the pad be too thin
galvanic action could arc between the two metals.

TIA,
Charlie Canitz, PE
Bel Air, MD